Andrew Coleman gives his final thoughts on the best use of GPS...

GPS satellite broadcasts are encoded on a high frequency carrier signal. Top of the range receivers use this carrier signal, not the messages contained within, to determine position.  The GPS carrier wavelength is over a thousand times smaller than the ‘normal’ timing signals so large improvements are theoretically possible.

However, it is extremely difficult to align these signals and sophisticated processing is required to minimise the errors.  So the equipment is very expensive and performance can vary as manufacturers have different proprietary solutions.  The very best systems give centimetre accuracy.

Surveying is the most common application, where a fixed base station does the hard work and rebroadcasts a correction signal to mobile units. This is known as Real Time Kinematics or RTK. The technology is also increasingly available in portable units. Innovation continues in this area and prices should come down.

A rough guide to pricing is to consider GPS correction solutions in four broad categories. Each offers roughly ten times greater accuracy for a tenfold increase in price: basic consumer GPS costs £20 for ± 10m, then £200, £2000 and £20,000 for ± 1cm.

How you use a GPS unit also affects performance. In general, the longer you run a GPS unit in one place, the greater its accuracy. So if you frequently move, lose signal or turn GPS off (i.e. to save power), you will risk lower accuracy or delays at each location. Models vary greatly in their start-up performance – known as ‘cold & warm starts’ – something to consider in specifications.

Finally, strictly speaking, GPS refers to the original USA military system, one of four Global Navigation Satellite Systems (GNSS). Of the others, Russia’s GLONASS is an option and Galileo, a new civilian GNSS developed by the EU and operational by 2014, looks the most exciting as it promises greater availability, accuracy and choice of commercial services.

GPS: what you really need to know:  

• Don’tbelieve the hype   
• Think what field accuracy you really need   
• Operating conditions of your GPS are important   
• Try before you buy, especially on big purchases   
• Don’t rely on GPS and plan for alternatives   
• Traditional location skills will still be needed  
•  In general you get what you pay for on performance, reliability and accuracy   
• Get independent advice